Eccentricity correction in a rolling mill

ABSTRACT

In a method of operating a rolling mill there is a measurement stage in which the rolls are rotated under load with the roll gap reduced to zero for a period of time equal to at least one beat cycle of the operative rolls (the work rolls in a two high mill and the back-up rolls in a four high mill) and a signal representative of the sum of the eccentricities of these rolls, and signals representative of the angular position of each of these rolls are supplied to a computer programmed to calculate and store, for each roll, data representative of the eccentricity at specific points spaced equally around its periphery. Following the measurement stage there is a correction stage during which material is rolled in the mill and signals representative of the sum of the eccentricities of the operative rolls are supplied from the computer in synchronism with the rotation of the rolls to roll gap adjusting means to adjust the roll gap so as to compensate for eccentricity of the rolls.

United States Patent 1 1 Clarke ECCENTRICITY CORRECTION IN A ROLLING MILL Robin Clarke, Sheffield, England [73] Assignee: Davy-Loewy Limited (formerly Davy and United Engineering Co.), Sheffield, England [22] Filed: Apr. 10, 1974 [21] App]. No.: 459,584

[75] Inventor:

[451 July 8,1975

Primary ExaminerMilton S. Mehr Attorney, Agent, or Firm-Brisebois & Kruger [57] ABSTRACT In a method of operating a rolling mill there is a measurement stage in which the rolls are rotated under load with the roll gap reduced to zero for a period of time equal to at least one beat cycle of the operative rolls (the work rolls in a two high mill and the back-up rolls in a four high mill) and a signal representative of the sum of the eccentricities of these rolls, and signals representative of the angular position of each of these rolls are supplied to a computer programmed to calculate and store, for each roll, data representative of the eccentricity at specific points spaced equally around its periphery. Following the measurement stage there is a correction stage during which material is rolled in the mill and signals representative of the sum of the eccentricities of the operative rolls are supplied from the computer in synchronism with the rotation of the rolls to roll gap adjusting means to adjust the roll gap so as to compensate for eccentricity of the rolls.

11 Claims, 2 Drawing Figures PATENTEDJUL 8 ms 3.893.317

LOAD REFERENCE Flal.

ECCENTRICITY CORRECTION IN A ROLLING MILL This invention relates to the operation of a rolling mill and in particular to a method of operating a rolling mill in order to compensate for the eccentricity of the mill rolls. Mill rolls. that is the work rolls in the case of a two-high mill and particularly the backup rolls in a four-high mill. these rolls being hereinafter referred to as the operative rolls. are eccentric to a certain degree about their roll necks and when the mill is in use a regular reoccurring pattern of gauge variation is imprinted on the material being rolled due to the eccentricity of the rolls. By eccentricity is meant any deviation of the roll periphery from a perfect cylinder about the axis of rotation.

Various proposals have been put forward in the past in order to deal with roll eccentricity and to compensate for it but none of the proposals have been particularly satisfactory or of a relatively simple nature and the compensation which has been possible under the known arrangements has left much to be desired.

In one known arrangement the eccentricity of the rolls is measured prior to rolling taking place and cccentric discs are manufactured whose eccentricity represents the eccentricity of the rolls in question to an enlarged scale and these discs are mounted on shafts which rotate synchronously and in a phase position corresponding with the eccentricity of the rolls to which they are connected. Some form of scanning device is associated with these discs to produce an electrical signal as the discs are rotated which is representative of the eccentricity of the rolls with which the discs are associated. it is clear that to provide these discs having the same eccentricity as the rolls is both a difficult and time consuming occupation and the final result is not satisfactory in that it is very unlikely that the eccentricity produced on the discs is exactly the same as that existing on the actual rolls.

It is an object of the present invention to provide a method of operating a rolling mill in order to compensate for the eccentricity of the mill rolls without cmploying either detectors directly monitoring the peripheral surfaces of the rolls or templates in which the cc centricity of the actual rolls is simulated.

According to the present invention in a method of operating a rolling mill in which during a measurement stage the rolls are rotated with the roll gap reduced to zero and the rolls under load for a period oftime equal to at least one heat cycle of the operative rolls and a signal representative of the sum of the cccentricities of these rolls and signals representing the angular position of each of these rolls are supplied to a computer programmed to calculate and store for each of the two operative rolls data representative of the eccentricity at specific points spaced equally around its periphery and subsequently during a correction stage with material being rolled between the rolls. signals representative of the sum of the eccentricitics of the two operative rolls are supplied from the computer in synchronism with the rotation of these rolls to roll gap adjusting means to bring about adjustment of the roll gap in the sense to compensate for the eccentricity of the operative rolls.

In the case ofa two-high rolling mill the eccentricity results from the two work rolls. In the case of a fourhigh mill however the bulk of the eccentricity results from the two back-up rolls and in this specification the work rolls of a two-high mill and the back-up rolls of a four-high mill are referred to as the operative rolls.

In order to apportion the source of the measured eccentricity between the two operative rolls it is essential that they rotate with significantly different angular velocities. The eccentricity components from the two rolls are each periodic but at slightly different frequencies due to their different angular velocities. Consequently the sum of the eccentricity components has a slow periodic heat and one heat cycle extends for many revolutions of the rolls. The term beat cycle in this specification is the period during which one roll. the roll having the smaller diameter. rotates through esactly one revolution more than the other roll.

During the first stage of the method of operating the rolling mill. that is the measurement stage in which eccentricity patterns are measured and stored. the eccentricity patterns are measured and stored in the computer separately for the two operative rolls and preferably they are also stored separately for the two ends of each roll giving a total of four stored patterns. During the measurement stage the measurement of the eccentricity signal is synchronised with the rotation ofthc operative rolls and therefore both ofthese rolls must have their angular position continuously measured. Various systems for accurately measuring the angular position of a roll may be used.

During the measurement stage the actuating means for adjusting the roll gap. which are preferably hydraulically operated. must be operated under constant load (or constant pressure) control. The roll eccentricities will then cause fluctuations in the length ofthe hydraulic actuating means and hence also in the length of linear displacement transducers associated therewith. It is the signals front these transducers which are used as a measure of the eccentricity.

During the correction stage. normal rolling applies and the hydraulic actuating means are usually position controlled (possibly including mill spring compensation. as in gaugen'i'eter type AGC). Correction signals are added to the position of the hydraulic actuating means in order tofcompensate for the mechanical eccentricity so that the length of the hydraulic actuating means varies in an equal and opposite manner to the sum of the stored changes in the radii of the operative rolls thus eliminating the changes in roll gap caused by eccentricity.

The mill can be'operated in a manner in which the stored eccentricity'patterns are not up-dated so that the correction will only be accurate so long as the mechanical eccentricity remains constant. If for example due to roll wear or thermal expansion the mechanical eccentricities change then it is necessary to repeat the measurement stage in order to restore accurate corrections.

Under certain circumstances it is possible for the measurement stage to be followed by a further stage which precedes the correction stage and in this further stage. with the roli=gap reduced to zero. corrections are applied to the roll gap adjusting means and at the same time further up-dated signals are stored in the computer ready for the correction stage. Again under certain operating conditions it is possible for this further stage to extend into the correction stage during which the stored data in the computer relevant to the eccentricity of the two operative rolls is continuously updated.

According to a second aspect of the invention a rolling mill has a pair of operative rolls. means for adjusting the roll gap. means for producing a signal representative of the sum of the eccentricities of the operative rolls when the rolls are rotated under load with the roll gap reduced to zero. means for producing signals representative of the angular position of each of the operative rolls and a computer connected to receive said signals and programmed to calculate and store for each of the operative rolls data representative of the eccentricity at specific points spaced equally around its periphery. the computer being arranged to subsequently supply signals representative of the sum of the eccentricities of the operative rolls in synchronism with the rotation of the rolls to the roll gap adjusting means.

In order that the invention may be more readily understood it will now be described. with reference to the accompanying drawings in which FIG. 1 is a diagrammatic view of a four-high rolling mill in accordance with the invention. and

FIG. 2 diagrammatically shows the eccentricity of the operative rolls of a mill.

Referring now to FIG. 1, a hydraulically controlled rolling mill consists of a pair of housings 2 each defining a window 4. The bottom back-up roll 6 is supported at its ends in chocks 8 which are supported in the window 4. The upper back-up roll is supported at its ends in chocks 12 which are displaceable by a pair of hydraulic rams 14 located in the housing windows and separated from the top of the window by a load cell 16. A pair of work rolls I8. are positioned between the back-up rolls 6 and 10.

Each hydraulic ram is supplied with hydraulic fluid through a separate servo valve 21 and the operation of the valves is controlled by a pair of controllers 22. The

displacement of the pistons 14a of the rams 14 is measured by separate linear displacement transducers 24 connected between the piston and the cylinder of each ram. The back-up rolls '6, 10 each have an angular position transducer 26 connected thereto.

The control for the operation of the rolling mill is identical for each side of the mill and for each side the controller 22 receives a load reference signal on line 28. a rolling load signal on the line 30 from the load cell 16 optionally a signal'from a pressure transducer PT anda signal from the linear displacement transducer on line 32; in addition the controller receives a signal on line 34 from a summing device 36 and an output signal from the controller is supplied to the servo valve 21 on line 38'. The controllers have two modes of operation (load control and position control).

A computer 40 receives signals from the angular position transducers 26, and from each of the transducers 24 and supplies signals to the summing devices 36 which also receive a position reference signal.

The computer deals with the two sides of the mill completely independently by using a time-sharing technique, although of course the roll angular position signals received from the transducers 26 are the same for both sides of the mill.

With no material in the roll gap and the work rolls brought into face contact and under load. the rolls are rotated at their normal speed. The signals from the linear displacement transducers 24 are fed into the com- 6 puter as are signals from the angular position transducers 26. This stage is referred to as a measurement stage and normally lasts for exactly one beat cycle although it may be advantageous to extend it for two or more complete beat cycles in order to improve accuracy and reduce the errors which arise from other random disturbances.

During the measurement stage the sum of the eccentricities of the two rolls is received by the computer as are signals representative of the angular position of the two rolls. The computer samples the signal representative of the sum of the eccentricities at time intervals corresponding to uniform increments of angular rotation of one of the rolls and the computer is programmed such that for each of the angular positions of the roll at which sampling takes place the value of the sample for that position at each of the revolutions of the roll during the beat cycle are averaged to provide data representative ofthe eccentricity of the roll at that angular position.

Referring now to FIG. 2. the upper graph shows the eccentricity X that is the deviation of the roll radius from its average value for the smaller of the two backup rolls plotted against time and the lower graph shows the eccentricity Y for the other roll plotted against time: The marks on the time axis represent complete revolutions of the rolls and one beat cycle is shown. comprising in an exaggerated example. six revolutions of the top roll and five revolutions of the bottom roll. Xl X6 are the eccentricity values at a single point on the top roll during successive revolutions. Y1 Y6 are the corresponding eccentricity values for the bottom roll. Since X1 X6 refer to the same point on the top roll X1 X2 X3 X6. Hence the average value of X1 X6 is equal to XI. The lower graph shows that Y! Y6 refer to six separate points equally spaced around the bottom roll. Assuming that any average DC level has been removed from Y so that Y now has an average of zero. it follows that the average of Y1 Y6 is approximately zero. This approximation improves as the number of points increases and it is reasonable to assume that the average of Y] Y6 equals zero. The input to the computer is the sum of X Y as shown for the points XI X6 and Yl Y6 in the lower graph of the drawing. The computer is programmed to take the average of the six readings it receives which is the average of X! Y1, X2 Y2 X6 Y6. This average is equal to the average of XI X6 plus the average of Y1 Y6. The average of X1 X6 is X] and the average of Y1 Y6 is zero so the result is XI. The computer has therefore calculated the value X1 for the top roll. By repeating the program for other points such as X7 the completeeccentricity pattern X for the top roll can be derived and similarly the eccentricity pattern Y for the bottom roll can be derived.

The stored eccentricity patterns effectively contain tables of deviation of back-up roll radius from its average value for a number of points equally spaced around the roll.

In the correction stage, that is when material is being rolled between the rolls. the values of the eccentricity of the two rolls are read from the stored tables in the computer in synchronism with roll rotation added together and sent from the computer as an analog correction signal. Readings from the tables can be taken either at times corresponding to the separate points around one of the rolls or at equal time intervals. The computer can interpolate between entries in the stored tables, to give a smoother output with improved resolution. Furthermore the computer can process the infor- 5 mation before it sends it out to give partial compensation for the dynamic lags in the hydraulic position con trol system.

Various optional features can enhance the basic system under suitable circumstances. especially for mills other than reversing hydraulic plate mills. Several options are listed below. i

1. During the measurement stage of the mill operation described above the mill is operated under constant load control and eccentricity information is obtained from the cylinder displacement transducers. As an alternative. the mill could be operated under constant position control (ie. fixed cylinder extension. as measured by the displacement transducers). but still with the rolls in contact and loaded. Eccentricity information would then be derived from the measured rolling load fluctuations (fed to the computer from the load cells 16 or pressure transducers). These load fluc' tuations can then be converted by the computer into corresponding displacements. using the mill spring constant (assumed known); analysis would then proceed as before.

2. For non-reversing mills. and especially for mills with a narrow speed range. a closed-loop method of eccentricity measurement should yield better accuracy. During a further stage which follows the measurement stage and precedes the correction stage the mill is operated under position control and the rolling load fluctuations are monitored (as in option I). but at the same time a provisional eccentricity correction pattern from the coomputer is sent out as a correction to the cylinder position reference. Initially the provisional correction could be either (a) zero. or (b) the eccentricity pattern previously measured in the measurement stage. The computer would then progressively refine the provisional eccentricity pattern. using information from the rolling load fluctuations. The aim is to reduce the load fluctuations to zero; thus any load fluctuations can be regarded as error signals in a closed-loop selfcorrecting system. Since the mill itself is included within the feedback loop. certain errors will be minimised'. these errors include gain errors in the computer inputs and outputs and in the hydraulic position control. and dynamic lags in the position control. Likewise. an error in the assumed value of mill spring constant will not be serious; it may take longer to achieve an ac curate measurement. but the accuracy ultimately achieved would not be sacrificed. The accuracy largely derives from the mill being operated under almost identical conditions in the measurement stage to those occurring in normal rolling.

3. For mills where the pass time is sufficiently long (e.g. strip mills), option 2 can be extended to continue the refining process on the stored eccentricity pattern even after strip is in the mill. This on-line updating process will enable slow changes in eccentricity (due. for example. to roll wear or thermal expansion) to be com tinuously incorporated into the stored eccentricity patterns. When strip is in the mill, load fluctuations will occur for reasons other than eccentricity (e.g. fluctuations in strip entry gauge). However. these will not be synchronised to back-up roll rotation. and hence they will cause little error provided the updating process is sufficiently slow.

4. Apparent eccentricity can be caused by hardness variations round the roll circumferences. giving rise to corresponding variations in roll flattening This apparcnt eccentricity will be dependent on the rolling load. and perhaps also on material width. To allow for this. two or more sets of aecentricity patterns could be stored within the computer. and interpolation (with respect to load) used to obtain the actual pattern corresponding to the known operation conditions ofthe mill. ln the case of load. the required sets of patterns could be derived by repeating the measurement stage two or more times under different load levelsv 5. The schemes could readily be extended to cater additionally for work roll eccentricity in a four high mill. If the work rolls are individually driven. their diameters must be maintained significantly different. and each must be fitted with angular position measurement. Work roll eccentricity measurement would then proceed in parallel with back-up roll eccentricity measu rement. It may be necessary to incorporate a correction routine into the computer to minimise interaction between the two measurements. lf. alternatively the work rolls have a common drive via a pinion box. then only one roll has to have angular position. measurement. and only one set of work roll patterns has to be stored (being the total eccentricity of both work rolls).

1 claim:

l. A method of detecting and correcting for the total eccentricity of two operative rolls in a rolling mill. which rolls differ slightly in diameter. lie on opposite sides of a gap through which a material is to be passed. and are responsible for the bulk of the eccentricity of all the rolls of said mill. said rolling mill comprising roll gap adjusting means and said method comprising the steps of rotating said rolls for a pe riod of time equal to at least one heat cycle in which the smaller of said rolls rotates through exactly one revolution more than the other of said rolls with the roll gap reduced to zero and the rolls under load. while continuously supplying signals representing the sum of the eccentricities of said rolls and signals representing the angular position of each of said rolls to a computer which is programmed to calculate and store for each of said rolls data representative of the eccentricity at specific points spaced equally around its periphery. and

subsequently rotating said rolls with said material being rolled between said rolls while supplying signals representative of said sum of said eccentricities from said computer in synchronism with said subsequent rotation of said rolls to said roll gap adjusting means to bring about adjustment of the roll gap in a direction which compensates for the eccentricity of both rolls.

2. A method as claimed in claim I in which the computer samples the signal representative of the sum of the eccentricities of the operative rolls at time intervals corresponding to uniform increments of angular rotation of one of the operative rolls and is programmed such that for each of the angular positions of the roll at which sampling takes place. the value of the sample for that position at each of the revolutions of said one roll during the beat cycle are averaged to provide data representative of the eccentricity of said one roll at said angular position.

3. A method as claimed in claim 1 in which separate signals representative of the sum of the eccentricity of the operative rolls are obtained for the two ends of the rolls and are supplied to the computer during the measurcment stage said signals being dealt with independently of each other by the computer. and during the correction stage separate control signals are supplied to roll gap adjusting means positioned at the two ends of the rolls.

4. A method as claimed in claim 1 in which the roll gap adjusting means comprise hydraulic rams positioned at the opposite ends of the rolls and during the measurement stage the rams are operated under constant load and displacement of the moveable parts of the rams serve to provide the signal representative of the sum of the ecccntricities of the operative rolls 5. A method as claimed in claim 1 in which the roll gap adjusting means comprise hydraulic rams positioned at the opposite ends of the rolls and during the measurement stage the rams are operated at constant length and variation of the rolling load serves to provide the signal representative of the sum of the eccentricitics of the rolls.

6. A method as claimed in claim Sin which the signal 1 is produced by a pressure transducer associated with the hydraulic supply to the ram.

7. A method as claimed in claim 1 in which after the measurement stage and prior to the correction stage, the measurement stage is repeated for at least one load lil on the operative rolls which is different from the load during the first measurement stage whereby data for at least two different loads on the rolls is stored in the computer and during the correction stage signals reprc sentative of the sum of the ecccntricities corresponding to the load on the rolls are calculated by interpolation between the separate sets of stored data and are supplied to the roll gap adjusting means.

8. A method as claimed in claim 5 in which during a further stage which follows the measurement stage and precedes the correction stage, the rolls are rotated under load with the roll gap reduced to zero and a correction signal provisionally representative of the eccen- 8 ti'icilies ol' the operative rolls is applied to the position control of the rams and a signal representative ot the uncorrected part of the sum of the ecccntricities of the operative rolls is supplied to the computer to update the provisional eccentricity data stored therein 9. A method as claimed in claim 8 in which the up dating of the stored data in the computer continues during the correction stage.

10. A method as claimed in claim 1 in which the mill is a four high mill and during the measurement stage the computer receives signals representative of the sum ofthe ecccntricities of the two work rolls and of the ecccntricities of the operative rolls. and signals representative of the angular position of at least one of the work rolls the computer calculates and stores for at least one of the work rolls data representative of the eccentricity at specific points spaced equally around its periphery and during the correction stage the roll gap adjusting means receives signals representative of the sum of the ecccntricities of the work rolls and of the ecccntricities of the operative rolls.

II. A rolling mill having a pair of operative rolls. means for adjusting the roll gap, means for producing a signal representative of the sum of the ecccntricities of the operative rolls when the rolls are rotated under load with the roll gap reduced to zero. means for producing signals representative of the angular position of each of the operative rolls and a computer connected to receive said signals and programmed to calculate and store for each of the operative rolls data representative of the eccentricity at specific points spaced equally around its periphery the computer being arranged to subsequently supply signals representative of the sum of the ecccntricities of the operative rolls in synchronism with the rotation ofthe rolls to the roll gap adjusting means. 

1. A method of detecting and correcting for the total eccentricity of two operative rolls in a rolling mill, which rolls differ slightly in diameter, lie on opposite sides of a gap through which a material is to be passed, and are responsible for the bulk of the eccentricity of all the rolls of said mill, said rolling mill comprising roll gap adjusting means and said method comprising the steps of rotating said rolls for a period of time equal to at least one beat cycle in which the smaller of said rolls rotates through exactly one revolution more than the other of said rolls with the roll gap reduced to zero and the rolls under load, while continuously supplying signals representing the sum of the eccentricities of said rolls and signals representing the angular position of each of said rolls to a computer which is programmed to calculate and store for each of said rolls data representative of the eccentricity at specific points spaced equally around its periphery, and subsequently rotating said rolls with said material being rolled between said rolls while supplying signals representative of said sum of said eccentricities from said computer in synchronism with said subsequent rotation of said rolls to said roll gap adjusting means to bring about adjustment of the roll gap in a direction which compensates for the eccentricity of both rolls.
 2. A method as claimed in claim 1 in which the computer samples the signal representative of the sum of the eccentricities of the operative rolls at time intervals corresponding to uniform increments of angular rotation of one of the operative rolls and is programmed such that for each of the angular positions of the roll at which sampling takes place, the value of the sample for that position at each of the revolutions of said one roll during the beat cycle are averaged to provide data representative of the eccentricity of said one roll at said angular position.
 3. A method as claimed in claim 1 in which separate signals representative of the sum of the eccentricity of the operative rolls are obtained for the two ends of the rolls and are supplied to the computer during the measurement stage said signals being dealt with independently of each other by the computer, and during the correction stage separate control signals are supplied to roll gap adjusting means positioned at the two ends of the rolls.
 4. A method as claimed in claim 1 in which the roll gap adjusting means comprise hydraulic rams positioned at the opposite ends of the rolls and during the measurement stage the rams are operated under constant load and displacement of the moveable parts of the rams serve to provide the signal representative of the sum of the eccentricities of the operative rolls.
 5. A method as claimed in claim 1 in which the roll gap adjusting means comprise hydraulic rams positioned at the opposite ends of the rolls and during the measurement stage the rams are operated at constant length and variation of the rolling load serves to provide the signal representative of the sum of the eccentricities of the rolls.
 6. A method as claimed in claim 5 in which the signal is produced by a pressure transducer associated with the hydraulic supply to the ram.
 7. A method as claimed in claim 1 in which after the measurement stage and prior to the correction stage, the measurement stage is repeated for at least one load on the operative rolls which is different from the load during the first measurement stage whereby data for at least two different loads on the rolls is stored in the computer and during the correction stage signals representative of the sum of the eccentricitIes corresponding to the load on the rolls are calculated by interpolation between the separate sets of stored data and are supplied to the roll gap adjusting means.
 8. A method as claimed in claim 5 in which during a further stage which follows the measurement stage and precedes the correction stage, the rolls are rotated under load with the roll gap reduced to zero and a correction signal provisionally representative of the eccentricities of the operative rolls is applied to the position control of the rams and a signal representative of the uncorrected part of the sum of the eccentricities of the operative rolls is supplied to the computer to update the provisional eccentricity data stored therein.
 9. A method as claimed in claim 8 in which the updating of the stored data in the computer continues during the correction stage.
 10. A method as claimed in claim 1 in which the mill is a four high mill and during the measurement stage the computer receives signals representative of the sum of the eccentricities of the two work rolls and of the eccentricities of the operative rolls, and signals representative of the angular position of at least one of the work rolls the computer calculates and stores for at least one of the work rolls data representative of the eccentricity at specific points spaced equally around its periphery and during the correction stage the roll gap adjusting means receives signals representative of the sum of the eccentricities of the work rolls and of the eccentricities of the operative rolls.
 11. A rolling mill having a pair of operative rolls, means for adjusting the roll gap, means for producing a signal representative of the sum of the eccentricities of the operative rolls when the rolls are rotated under load with the roll gap reduced to zero, means for producing signals representative of the angular position of each of the operative rolls and a computer connected to receive said signals and programmed to calculate and store for each of the operative rolls data representative of the eccentricity at specific points spaced equally around its periphery, the computer being arranged to subsequently supply signals representative of the sum of the eccentricities of the operative rolls in synchronism with the rotation of the rolls to the roll gap adjusting means. 